The United States called on China to end "business as usual" with its ally North Korea after Pyongyang defied world powers by saying it had tested a hydrogen bomb, while South Korea prepared to retaliate by broadcasting propaganda across the border.

South Korea, which has grown increasingly close to China in recent years, said its foreign minister would speak with his Chinese counterpart today.

US Secretary of State John Kerry said he had made clear in a phone call with Chinese Foreign Minister Wang Yi that China's approach to North Korea has not succeeded.